Purge Valve Question

DBone · #1 01-07-2009, 12:34 PM

I'm totally new to FI, so I'm trying to get my hands around what I need and don't need.

I had a deposit in at Superior for an IO-360 with the Precision Silverhawk FI. From what I remember figuring out at the time (a while ago), was that I didn't need any special plumbing and definitely nothing back to the tanks. I talked to Andy at Andair, and he knew exactly which fuel selector valve I needed to make the whole system work.

Fast forward to today, and I am most likely going with a Lycoming IO-360 with an Air Flow Performance system, and I am reading about the purge valve setup. I left a vm for the folks at AFP, but I thought I'd ask hear as well.

Will I need to buy any additional components to make the AFP system (with the purge valve) work, will I have additional components that I need to get rid of, or will what I have handle the new setup?

Thanks for helping a newbie out!

Mike S · #2 01-07-2009, 02:13 PM

You should be fine with what you have, you will need to add a return line, this usually goes back to one of the tanks.

The return line is not usually valved, as it is not a continuous flow----just when in the purge position.

You could just plumb it back into one of the feed lines, but that will defeat the idea of running cool tank fuel through the heat soaked system on a hot start.

Give Don a call at AFP, good guy and very helpfull.

Good luck

erich weaver · #3 01-07-2009, 03:02 PM

You wont need Vans control bracket (VA-182?), as this is supplied by AFP. Others (me, other Checkoway followers), have plumbed the purge line back into one of the supply lines instead of back to a tank. If you purge to the supply line from the right tank (for example), and set the fuel selector to the left tank, you wont be recirculating hot fuel.

erich

flion · #4 01-07-2009, 06:42 PM

The purge valve is only used in certain circumstances anyway and you will probably not be running it enough to recirculate hot fuel. As for the mixture bracket thing, I found that it was difficult to route on my -6A and it looks like you will have similar fun with the -7. I'm assuming vertical induction? Anyway, here's my approach, complete with pictures.

N184JG · #5 01-08-2009, 11:58 AM

I use the purge valve for every start and every shut down per AFP.

airguy · #6 01-08-2009, 05:21 PM

Quote:

Originally Posted by N184JG

I use the purge valve for every start and every shut down per AFP.

Shutdown, I can see. Every start just doesn't make sense - you only need it for hot starts or any other situation where the fuel on top of the engine in the divider and injector lines has been heated.

frankh · #7 01-08-2009, 06:17 PM

With wingrrot fuel pumps you can simply turn off the pumps until the engine quits..I agree, purge for hot starts only

Frank

Bob Brown · #8 01-08-2009, 07:55 PM

I use my purge valve for every shutdown, it works much better than the mixture control, the engine just stops right now.
As far as hot starts, it depends on the hot start...if you are starting right up like say after a quick fuel stop, I just start cranking as I push in the purge cable...usually the engine is running right as its completely closed (allowing fuel to flow to the divider).
If I've been sitting awhile and its a hot day, I'll go through the purge routine.
My 1/4" return line is tee'd into the left tank line ala Checkoway, so I always use the right tank for purge valve hot starts...works great.

N184JG · #9 01-08-2009, 09:19 PM

Quote:

Originally Posted by airguy

Shutdown, I can see. Every start just doesn't make sense - you only need it for hot starts or any other situation where the fuel on top of the engine in the divider and injector lines has been heated.

You don't understand the purge valve at all . Its not used just for hot starts it works much better then the mixture for starting a cold engine. With the fuel pump on close the purge valve to prime the engine then open crank engine as soon as engine starts close purge valve mixture is full rich all the time. This is per AFP.

airguy · #10 01-08-2009, 09:26 PM

Quote:

Originally Posted by N184JG

You don't understand the purge valve at all . Its not used just for hot starts it works much better then the mixture for starting a cold engine. With the fuel pump on close the purge valve to prime the engine then open crank engine as soon as engine starts close purge valve mixture is full rich all the time. This is per AFP.

I beg to differ. I do understand the purge valve, both it's function and it's operational purpose. My point is that it's not needed for a cold start. Using it is no detriment, but neither it is a great advantage, and it's certainly not required. The purge valves specific purpose is to recirculate fuel from the flow divider back to the fuel supply, (hence the name PURGE valve) which has the side effect of dumping all the pressure on the injector lines which allows it to be used as an excellent idle cutoff tool. The whole operational point for the existence of the purge valve is to solve hot start problems by removing heated fuel from the FWF area.